This invention relates to chemical reactions performed in the presence of Lewis acids as catalysts. More particularly, it relates to a genus of compounds useful as such catalysts.
Numerous types of chemical reactions proceed best when catalyzed by Lewis acids. By "Lewis acid" is meant a compound that can accept an electron pair. Numerous types of Lewis acids are known, and many of them are transition metal compounds, especially the halides. Illustrative compounds are ferric chloride, aluminum chloride, zinc chloride and stannous chloride.
Lewis acid-catalyzed reactions include many forms of esterification. For example, the reaction of an aromatic carboxylic acid with an alcohol is best conducted in the presence of a Lewis acid. The same is true of the reactions of carboxyiic acids with epoxides. In each case, the product is an ester of the carboxylic acid.
It is of interest to develop new kinds of Lewis acids, particularly in the form of organic compounds. Since many esterification reactions take place at relatively high temperatures, Lewis acid-active organic compounds which are stable under high temperature conditions are especially desirable.
Hexaaikylguanidinium salts are known, as is their use as phase transfer catalysts is known. As disclosed, for example, is U.S. Pat. Nos. 5,116,975 and 5,229,482, they may be employed in various reactions ultimately yielding polyimides as products. They are also known for their high thermal stability when so used.